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Dimerization of eosin on nanostructured gold surfaces: Size regime dependence of the small metallic particles
Chemical Physics Letters 412 (2005) 5–11 www.elsevier.com/locate/cplett
Dimerization of eosin on nanostructured gold surfaces: Size regime dependence of the small metallic particles Sujit Kumar Ghosh a, Anjali Pal b, Sudip Nath a, Subrata Kundu a, Sudipa Panigrahi a, Tarasankar Pal * b
a Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, West Bengal, India Civil Engineering Department, Indian Institute of Technology, Kharagpur-721302, West Bengal, India
Received 24 February 2005; in final form 13 June 2005 Available online 12 July 2005
Abstract Gold nanoparticles of variable sizes have been exploited to study their influence on the absorption and emission spectral characteristics of eosin, a fluorescent dye. It has been found that smaller particles of gold stimulate J-aggregation of eosin on the surface of metal particles whereas larger particles cannot induce any kind of aggregation amongst the dye molecules. The size regime dependence of the gold nanoparticles has been attributed to the intercluster interactions induced by the dye molecules for smaller gold nanoparticles and consequently, close packing of the dye molecules around the gold surface engenders intermolecular interactions amongst the dye molecules leading to dimerization. 2005 Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable research efforts have been undertaken to investigate the photophysical and photochemical aspects of multicomponent nanostructured assemblies consisting of metals and electroactive/ photoactive dyes [1,2]. Association of the electroactive dye molecules onto the surface of metallic nanoparticles very often leads to aggregation effects [3]. Such organic– inorganic hybrid moieties have numerous possible applications in developing efficient light energy conversion systems, optical devices, and sensors. The composites of dye aggregate and metal nanoclusters find applications in biomolecule sensing and imaging applications [4]. Fundamental understanding of such events becomes invaluable guidance for practical applications. Theoretical studies have shown that the nature of molecular orientation of the dye molecules in the aggregate deter*
Corresponding author. Fax: +91 03222 255303. E-mail address: [email protected] (T. Pal).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.059
mines the spectral shift in the absorption band [5]. The nature of orientation of the dye molecules governs the type of dye aggregates which exhibit either a blue shift (H-type) or red shift (J-type) in the extinction spectra. Since the formation of aggregates alters the absorption spectrum and photophysical properties of dyes, the resulting aggregates could emit at a wavelength different from their monomer or show new photosensitizing properties. In addition to their use as photographic sensitizers, the large oscillator strength and fast electronic response of J-aggregates are of interest for modeling energy transfer in photosynthetic reaction center antenna, non-linear optics related to superfluorescence, and solar photochemical energy conversion [6]. During the past few years, significant research interest has been devoted to modify the surface of metal colloids with organic dyes. Kamat et al. [3] reported efficient capping of SnO2 and SiO2 matrices with Haggregates of rhodamine 6G. They have extended their studies taking colloidal gold into consideration and explained the dye aggregation in terms of intermolecular
6
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
interactions [7]. Kerker [8] reported that for silver particles coated with Rhodamine 6G, a blue-shift was observed for both silver surface plasmon band and visible absorption band of the dye. Recently, the Jaggregation of cyanine dyes on the surface of noble metal colloids has been reported [9]. Wiederrecht et al. [6] showed that photoexcitation of the plasmon in Ag nanoparticles coated with J-aggregates of a cyanine derivative leads to exciton dynamics that are much different than for J-aggregate monolayers on bulk metal surfaces. Metal nanoclusters (copper, silver, and gold) embedded in a copper phthalocyanine matrix have been shown to enhance non-linear optical processes [10]. In a recent communication [11], it has been shown that by using an amine tethering group it is possible to organize 1-methylaminopyrene chromophores on to gold nanoparticles. Makarova et al. [12] and Templeton et al. [13] modified the surfaces of gold nanoparticles with fluorescein isothiocyanate. They showed that dye molecules chemisorbed on the gold surface do not induce any aggregation. Wan et al. [14,15] examined the dimerization of halofluorescein dyes on Au(1 1 1) electrode in aqueous HClO4 solution by cyclic voltammetry and in situ scanning tunneling microscopic studies. Eosin yellowish [2 0 ,4 0 ,5 0 ,7 0 -tetrabromo-3 0 ,6 0 -di-hydroxyspiro[isobenzofuran-1(3H),9 0 -[9H]xanthen]-3-one] is a xanthene dye with strong absorption in the visible region. A number of well-characterized photophysical and photochemical properties of the dye molecules have enabled several researchers to study their spectral features in a variety of microenvironments. In this Letter, we have investigated the spectral characteristics of eosin in the presence of eight different sizes of gold nanoparticles. It has been found that smaller particles of gold stimulate J-aggregation of eosin on the surface of metal particles whereas any kind of aggregation amongst the dye molecules is not seen in the presence of larger particles. The changes in the spectral characteristics of eosin due to aggregation on metallic nanoparticles have been observed both in absorption and emission spectroscopy. Therefore, the present report addresses the behavior of eosin on the wide range of nanosized gold surfaces and probes the aggregation effect in a dye–metal nanocluster assembly.
COOH Br
Br O H
O
O Br
Br Eosin
2. Experimental 2.1. Reagents and instruments All the reagents used were of AR grade. Chloroauric acid (HAuCl4 Æ xH2O) and non-ionic surfactant, poly (oxyethylene) isooctyl phenyl ether (Triton X-100 or TX-100) were purchased from Aldrich. Molecular probe, eosin (Sigma) and ascorbic acid (SRL, India) were used as received. Doubled distilled water was used throughout the course of investigation. All chemical reactions were carried out in 1 cm wellstoppered quartz cuvettes. For the preparation of gold seeds, photoirradiation was carried out with a germicidal lamp (Philips, Holland, G15 T8 UVC, 15 W). The absorption spectra were measured in a Spectrascan UV 2600 spectrophotometer (Chemito, India). Fluorescence spectra were recorded with a Perkin–Elmer LS50B spectrofluorimeter equipped with a 9.9 W xenon flash lamp and a photo multiplier tube with S-20 spectral response. An excitation wavelength of 500 nm and slit width of 5/5 nm were used to record the spectra. Transmission electron microscopic (TEM) studies were carried out in a JEOL 4000 instrument at 400 kV. Samples were prepared by placing a drop of solution on a carbon coated copper grid. The temperature was 298 ± 1 K for all measurements. 2.2. Preparation of gold nanoparticles of variable sizes Gold nanoparticles were prepared by a two-step seedmediated (non-iterative) growth method. Seed particles were prepared by the UV-irradiation of a solution containing Au(III) ions (added as HAuCl4 Æ xH2O) in aqueous solution of TX-100. In practice, an aqueous solution (3 mL) of HAuCl4 (0.1 mM) was mixed with TX-100 (10 mM) in 1 cm quartz cuvette and photoirradiated for 30 min. The cuvette was kept at a distance of 3 cm from the light source. The particles formed by this method serve as seed particles (S). After 3–4 h appropriate amounts of seed particles and Au(III) ions were mixed in different molar ratios and to this solution ascorbic acid solution (0.13 mM) was added all at a time. This resulted in various sized gold particles depending on the seed particle to Au(III) concentration ratio. The particles formed by this method were allowed to stand for 24 h before their use for further studies. An in-depth study of the evolution and characterization of the gold particles has been discussed elsewhere [16]. The UV– vis spectral characteristics and sizes of the gold nanoparticles are summarized in Table 1. 2.3. Making gold–eosin assembly A known concentration of aqueous solution of eosin (7.5 lM) was mixed with the gold colloids (10 lM) of
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
7
Table 1 Synthetic conditions and UV–visible spectrophotometric and transmission electron microscopic characteristics of the prepared gold particles Sets
[seed Au] (M)
[Au(III)] (M)
kmax (nm)
Bandwidth (nm)
Particle diameter (nm)
S A B C D E F G
1.0 · 104 7.5 · 105 6.0 · 105 5.0 · 105 3.33 · 105 1.00 · 105 3.33 · 106 1.00 · 106
– 2.50 · 105 4.00 · 105 5.00 · 105 6.67 · 105 9.00 · 105 9.67 · 105 9.90 · 105
522 524 526 527 527 529 531 534
70 68 64 63 55 50 47 47
10
Exp.
different sets and the final volume of all the solutions were made to 3 mL. The mixtures were then allowed to stand for 12 h (for the reproducible dimerization) and subjected to absorption and emission measurements. All the mixtures contained 1 mM (300 lL of 10 mM) of TX-100 solution.
3. Results and discussion Aqueous solution of eosin (7.5 lM) exhibits a sharp monomeric absorption band in the visible region with a maximum at 516 nm (trace a) as shown in Fig. 1. In a micellar solution of TX-100 (1.0 mM), the absorption maximum is slightly red-shifted (kmax 518 nm) compared to that of the simple aqueous solution (trace b). However, significant alteration in the absorption characteristics of the dye (trace c) is seen in the presence of gold colloids (10 lM, set S). Keeping these observations in mind, we intend to study the size effect of gold nanoparticles on eosin taking the spectral features of the dye molecules into
a
0.6
b Absorbance (a. u.)
c 0.4
0.2
0.0 450
500 550 Wavelength (nm)
600
Fig. 1. Absorption spectra of eosin (7.5 lM) in (a) aqueous, (b) micellar solution (1 mM) and (c) in the presence of gold colloids (10 lM, set S). The gold colloid contains 1 mM of TX-100 solution.
12 14 22 31 46
Cal. 11 11.9 12.6 14.4 21.5 31.1 46.4
consideration. For this purpose, gold nanoparticles of eight different sizes have been employed. The size of the particles varies within the size range of 10–46 nm (sets S–G) as listed in Table 1. Interestingly, it is observed that the absorption characteristics of eosin show two distinct features for the two different size regimes. The amount of total gold (10 lM) is kept same for all the sets. The concentration of the dye is kept constant (7.5 lM) in all cases and all the solutions contain 1 mM of TX-100 solution. Fig. 2a shows the behavior of eosin in the absence and presence of different sets of gold particles. For gold particles (sets S, A, B, C) lying in the size range of 10–12 nm, a red shift (20 nm) in the absorption spectra of the dye are seen with a maximum at 538 nm. However, the band structures remain same in the presence of all sets of particles. As the size of the gold particles (sets D, E, F, G) become larger than 12 nm, the absorption maxima of eosin appears at 518 nm and the absorbance value slightly decreases. The optical properties of the dye molecules changes upon adsorption onto metal particles because the local field at the molecule increases by the dipole field of the resonant plasma sphere. The strong electron coupling between the dye and metallic particles results in the decrease in the intensity of the eosin absorption peak upon adsorption onto gold particles [8]. Eosin, in its monomeric form, is a strongly fluorescing dye (uf = 0.57). In the presence of gold particles, the emission behavior of the dye changes. Fig. 2b shows the fluorescence spectra of eosin in the absence and presence of gold nanoparticles. Eosin exhibits a broad emission band with a maximum at 544 nm (trace a*) in the presence of TX-100. Addition of colloidal gold (sets S, A, B, C) to the eosin solution lead to shifting of emission maxima to 557 nm as shown by the traces s, a, b, c respectively. In the presence of larger gold particles (sets D, E, F, G), the emission maxima again appear at 544 nm and the emission intensity decreases. The emission spectra of eosin in the presence of sets D, E, F, G are shown by traces d, e, f, g, respectively. A small variation in the intensity of eosin emission in the presence of different sets of gold particles is clearly resolved in
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S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
Fig. 2. (a) Absorption spectra of eosin (7.5 lM) in the absence and presence of gold particles of variable sizes (10 lM). (b) Emission spectra of eosin (7.5 lM) in (a*) absence and (S–G) presence of gold particles of variable sizes (10 lM).
the fluorescence spectroscopy (not distinguishable from the absorption spectroscopy). The appearance of red-shifted band at 538 nm in the extinction spectra can be attributed to the dimeric structure of the dye [6,9]. The red shift in the absorption maximum observed in Fig. 2a indicates that the eosin aggregates formed in colloidal suspensions are J-type. In fact, the J-aggregates are characterized by a redshifted absorption band relative to the monomer band as a result of exciton delocalization over the molecular building blocks of the aggregate [17]. The molecular exciton theory was proposed by McRae and Kasha [5] to afford the most satisfactory treatment of the electronic transitions in the dimeric form of the dye. This theory predicts that the excitonic singlet state of the dimer splits into two levels as a consequence of the two possible arrangements (in-phase and out-of-phase oscillation) of the transition dipoles of the chromophores in
the dimer. Since the exciton splitting depends on the oscillator strength of the transition, only the singlet excited state splits and the triplet excited state remains nearly degenerate. As a result of splitting, one level is now lower and the other is higher in energy than the corresponding monomer singlet excited state. Transitions from ground state to either excited state are possible and the number of bands actually observed depends on the geometry of the dimer. For parallel dimers (Htype aggregates), the transition to the lower energy excited state is forbidden and a single blue-shifted band is observed with respect to monomer. On the other hand, in case of head-to-tail dimers (J-type aggregates), the transition to the higher energy excited state is forbidden and the spectrum shows a single red-shifted band with respect to monomer. A schematic presentation of the H- and J-type aggregates and representative energy levels that control the excited state dynamics [3] are shown in Scheme 1. Kamat et al. [7] observed a blue shift in the absorption maximum upon addition of Rhodamine 6G to the gold colloids and concluded that the dimeric aggregates are of H-type. They pointed out a strong electronic coupling between the Rhodamine 6G molecules in the presence of gold colloids even at the very low (<105 M) dye concentrations. Since eosin is a strongly fluorescing dye, the aggregation effect is also seen in the emission spectra of the dye molecules. Absorption and emission spectroscopy are the techniques most widely used and yield the most important information about the aggregation of dye molecules in solution. Although Lo´pez Arbeloa reported the existence of trimer and higher aggregates in concentrated aqueous solution for such halofluorescein dyes [18], the presence of a single absorption band (kmax 538 nm) is indicative of the fact that only one kind of S1+ ic
S1 isc
_ S1
isc T1
T1
hv
E
E
hv //
hv /
E E
E
H-type
E
J-type S0
S0 Monomer
Dimer
Scheme 1. Representation of energy levels (S, singlet; T, triplet) of eosin (E) monomer and dimer that control the excited state dynamics [3].
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
aggregate exists on the surface of the gold colloids. It is necessary to recall that ascorbic acid is used for the growth of the gold particles and the absorption changes of eosin may occur due to some kind of interaction of the dye molecules with the unreacted ascorbic acid or the oxidation product of ascorbic acid (produced in the reaction medium during the reduction of gold chloride) molecules. However, no such changes are observed in the absorption spectrum when the same amount of ascorbic acid is added to eosin solution which indicates that shifting of absorption maximum is not due to unreacted ascorbic acid molecules. Furthermore, the shifting of dye maximum is also seen in the presence of seed particles (S) that contain no ascorbic acid or the oxidation product of ascorbic acid molecules. Fluorescence decay measurements (kex 405 nm) of excited dye molecules exhibit lifetimes of 3.4 and 3.2 ns in micellar environments and the presence of gold particles indicating the attachment of the probe molecules on the gold surface. As the dye aggregation does not occur at such a low concentration of the dye in the absence gold particles, it is reasonable to assume that intermolecular interactions between the eosin molecules on the gold surface is responsible for the dimerization process. Since in the presence of gold colloids the dye kmax appears at 538 nm, it is suspected that the gold nanoparticles can contribute to the absorption spectrum of the dye. But it was noted that for such a low concentration of the gold colloid (10 lM), the optical density is negligibly small and can be neglected compared to dye absorption. Moreover, the shifting of dye emission maximum is also seen in the fluorescence spectrum of the dye, but the gold nanoparticles are non-fluorescent. Therefore, it can be concluded that the shifting of dye absorption is not due to the interaction with the ascorbic acid molecules or to significant optical contribution due to the gold particles. The size regime dependence of the gold nanoparticles on the dimerization of eosin molecules can be rationalized by correlating the structure of the nanoparticles in terms of their surface parameters. The noticeable shift of the absorption and emission maximum of eosin is seen only in the presence of smaller gold nanoparticles that for a specified size regime but not for particles of larger dimensions. As the gold concentration is same for all sets of particles, the larger is the particle the lower is the concentration of the particles and the surface area is correspondingly lower. The number of dye molecules associated with each gold nanoparticle was determined by calculating the particle concentration in the gold colloids. At first, the aggregation number was calculated using the formula: NAu = (59 nm3)(p/6)(DMS)3, where DMS is the mean diameter of the particles. Thus, a gold particle of 10 nm diameter is composed of 30 867 gold atoms and each particle corresponds to 23 163 eosin mole-
9
cules. The surface-to-volume atomic ratio (a) of metal particles is defined by, a = 3d/R, where d and R denote the atomic diameter and particle size respectively. ˚ for atomic gold, the surfaceAssuming d = 3.58 A to-volume atomic ratio, a = 0.1074 which indicates amongst the 30 867 gold atoms in a 10 nm gold particle, 3315 atoms are located at the surface. It has been seen that the formation of multilayer of the dye molecules on metallic nanoparticles depends on dye concentration, pH and hydrogen bonding [19]. Thus, a 10 nm gold particle with a surface area of 314 nm2 and 3315 atoms on the surface accommodate large number of eosin molecules through the formation of multilayers of the dye molecules which contain functional moieties capable of forming hydrogen bonds. The number of eosin molecules (the dye concentration remaining same for all sets) corresponding to the gold surface increases with increase in particle size as the number of particles decreases in solution. Thus, higher dye concentrations per gold nanoparticle existing in solutions through D–G and is expected to induce more extensive dimerization than in colloids S–C. The observed dye aggregation in the presence of smaller particles can be interpreted by considering that dye molecules induce intercluster interactions within the smaller particles and such aggregates which bring adsorbed dye molecules closer facilitate dimer formation in the assembly of the dye molecules [20]. Since, the molar surface area increases with decreasing particle size, there results an increase in molar free energy of the small particles compared to larger ones and thus, the nanoparticle aggregation predominates in colloids with smaller particles after addition of the dye [21]. So, aggregation of particles is the cause and dimerization of dye molecules is the effect. Transmission electron micrographs (Fig. 3) of the gold colloids (set S) before and after addition of the dye molecules show that the gold particles are aggregated in solution after addition of the dye molecules. During the formation of gold aggregates, the close packing of the dye molecules around the gold surface induce interparticle interactions and dipolar interaction between the two bromine atoms from two eosin molecules is responsible for the dimer formation [14,15]. Fig. 4 shows an increase in the intensity of 538 nm peak, i.e., increases the concentration of the dimer with increase in the concentration of gold colloids (set S). Inset shows a linear increase of the dimer concentration with increase in the concentration of the gold colloids indicating that extent of dimerization increases with increased number of gold particles in the aggregate. As the dimerization of the dye molecules occurs due to aggregation amongst the gold particles, an aging (12 h) of the gold–probe assembly is required for the reproducible dimerization of the dye molecules. The formation of dye aggregates only in the presence
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S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
Fig. 3. Transmission electron micrograph of the gold particles (set S) (a) before and (b) after addition of the dye molecules.
cles due to their high molar surface free energy that the surface phase possesses. The formation of dye aggregates modifies the absorption and emission spectrum and the photophysical properties of the dye. Therefore, the aggregation phenomenon of eosin can be successfully exploited as a marker to label different size regimes of gold nanoparticles utilizing absorption and fluorescence spectroscopy.
0.6
0.4
Absorbance (a. u.)
Absorbance (a.u.)
0.52
0.48
0.44
0.40 0
2
4
6
8
10
[Au] (µM)
0.2
Acknowledgments
0.0 400
450
500 Wavelength (nm)
550
600
Fig. 4. Extinction spectrum of eosin (7.5 lM) in presence of different concentration of gold particles (set S). Inset shows the plot of absorbance (at 538 nm) as a function of gold concentration.
of smaller particles indicates that there is a dramatic increase in surface energy of the particles as the diameter decreases below 12 nm. Recently, it has been reported from our laboratory that fluorescence quenching of 1-methylaminopyrene and the catalytic reduction of eosin on the surface of gold nanoparticles are also size regime dependent phenomena [16,22].
4. Conclusion This Letter describes a straightforward experimental but spectroscopic verification of size effect of nanoscale materials on the dimerization of eosin molecules. The dye molecules undergo dimerization when bound to smaller gold particles even at very low concentrations but remain in its monomeric form in the presence of larger gold nanoparticles. The major factor driving the dimerization process is the aggregation of smaller parti-
We gratefully acknowledge financial support from DST and CSIR, New Delhi. We are thankful to Prof. Kankan Bhattacharyya for providing DST funded Femtosecond Facilities for lifetime measurements.
References [1] M. Kawasaki, T. Sato, Y. Yoshimoto, Langmuir 16 (2000) 5409. [2] R. Shinozaki, T. Nakato, Langmuir 20 (2004) 7583. [3] C.P. Nasr, D. Liu, S. Hotchandani, P.V. Kamat, J. Phys. Chem. 100 (1996) 11054. [4] P. Xu, H. Yanagi, Chem. Mater. 11 (1999) 2626. [5] E.G. McRae, M. Kasha, in: L. Augenstein, R. Mason, B. Rosenberg (Eds.), Physical Processes in Radiation Biology, Academic Press, New York, 1964, p. 23. [6] J. Hranisavljevic, N.M. Dimitrijevic, G.A. Wurtz, G.P. Wiederrecht, J. Am. Chem. Soc. 124 (2002) 4536. [7] N. Chandrasekharan, J. Hu, G. Jones II, P.V. Kamat, J. Phys. Chem. B 104 (2000) 11103. [8] M. Kerker, J. Colloid Interface Sci. 105 (1985) 297. [9] N. Kometani, M. Tsubonishi, T. Fujita, K. Asami, Y. Yonezawa, Langmuir 17 (2001) 578. [10] O. Stenzel, A. Stendal, A.C. Voigtsberger, Solar Energy Mater. Sol. Cells 37 (1995) 337. [11] K. Goerge Thomas, P.V. Kamat, J. Am. Chem. Soc. 122 (2000) 2655. [12] O.V. Makarova, A.E. Ostafin, H. Miyoshi, J.R. Norris, D. Miesel, J. Phys. Chem. B 103 (1999) 9080. [13] A.C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27. [14] G.-J. Su, S.-X. Yin, L.-J. Wan, J.-C. Zhao, C.-L. Bai, Chem. Phys. Lett. 370 (2003) 268.
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11 [15] G.-J. Su, S.-X. Yin, L.-J. Wan, J.-C. Zhao, C.-L. Bai, Surf. Sci. 551 (2004) 204. [16] T.K. Sau, A. Pal, T. Pal, J. Phys. Chem. B 105 (2001) 9266. [17] D.A. Higgins, P.J. Reid, P.F. Barbara, J. Phys. Chem. 100 (1996) 1174. [18] I. Lo´pez Arbeloa, J. Chem. Soc., Faraday Trans. 2 (77) (1981) 1725.
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[19] E.W.L. Chan, D.C. Lee, M.K. Ng, G. Wu, K.Y.C. Lee, L. Yu, J. Am. Chem. Soc. 124 (2002) 12238. [20] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [21] H. Zhang, R.L. Penn, R.J. Hamers, J.F. Banfield, J. Phys. Chem. B 103 (1999) 4656. [22] S.K. Ghosh, A. Pal, S. Kundu, S. Nath, T. Pal, Chem. Phys. Lett. 395 (2004) 366.
Dimerization of eosin on nanostructured gold surfaces: Size regime dependence of the small metallic particles Sujit Kumar Ghosh a, Anjali Pal b, Sudip Nath a, Subrata Kundu a, Sudipa Panigrahi a, Tarasankar Pal * b
a Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, West Bengal, India Civil Engineering Department, Indian Institute of Technology, Kharagpur-721302, West Bengal, India
Received 24 February 2005; in final form 13 June 2005 Available online 12 July 2005
Abstract Gold nanoparticles of variable sizes have been exploited to study their influence on the absorption and emission spectral characteristics of eosin, a fluorescent dye. It has been found that smaller particles of gold stimulate J-aggregation of eosin on the surface of metal particles whereas larger particles cannot induce any kind of aggregation amongst the dye molecules. The size regime dependence of the gold nanoparticles has been attributed to the intercluster interactions induced by the dye molecules for smaller gold nanoparticles and consequently, close packing of the dye molecules around the gold surface engenders intermolecular interactions amongst the dye molecules leading to dimerization. 2005 Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable research efforts have been undertaken to investigate the photophysical and photochemical aspects of multicomponent nanostructured assemblies consisting of metals and electroactive/ photoactive dyes [1,2]. Association of the electroactive dye molecules onto the surface of metallic nanoparticles very often leads to aggregation effects [3]. Such organic– inorganic hybrid moieties have numerous possible applications in developing efficient light energy conversion systems, optical devices, and sensors. The composites of dye aggregate and metal nanoclusters find applications in biomolecule sensing and imaging applications [4]. Fundamental understanding of such events becomes invaluable guidance for practical applications. Theoretical studies have shown that the nature of molecular orientation of the dye molecules in the aggregate deter*
Corresponding author. Fax: +91 03222 255303. E-mail address: [email protected] (T. Pal).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.059
mines the spectral shift in the absorption band [5]. The nature of orientation of the dye molecules governs the type of dye aggregates which exhibit either a blue shift (H-type) or red shift (J-type) in the extinction spectra. Since the formation of aggregates alters the absorption spectrum and photophysical properties of dyes, the resulting aggregates could emit at a wavelength different from their monomer or show new photosensitizing properties. In addition to their use as photographic sensitizers, the large oscillator strength and fast electronic response of J-aggregates are of interest for modeling energy transfer in photosynthetic reaction center antenna, non-linear optics related to superfluorescence, and solar photochemical energy conversion [6]. During the past few years, significant research interest has been devoted to modify the surface of metal colloids with organic dyes. Kamat et al. [3] reported efficient capping of SnO2 and SiO2 matrices with Haggregates of rhodamine 6G. They have extended their studies taking colloidal gold into consideration and explained the dye aggregation in terms of intermolecular
6
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
interactions [7]. Kerker [8] reported that for silver particles coated with Rhodamine 6G, a blue-shift was observed for both silver surface plasmon band and visible absorption band of the dye. Recently, the Jaggregation of cyanine dyes on the surface of noble metal colloids has been reported [9]. Wiederrecht et al. [6] showed that photoexcitation of the plasmon in Ag nanoparticles coated with J-aggregates of a cyanine derivative leads to exciton dynamics that are much different than for J-aggregate monolayers on bulk metal surfaces. Metal nanoclusters (copper, silver, and gold) embedded in a copper phthalocyanine matrix have been shown to enhance non-linear optical processes [10]. In a recent communication [11], it has been shown that by using an amine tethering group it is possible to organize 1-methylaminopyrene chromophores on to gold nanoparticles. Makarova et al. [12] and Templeton et al. [13] modified the surfaces of gold nanoparticles with fluorescein isothiocyanate. They showed that dye molecules chemisorbed on the gold surface do not induce any aggregation. Wan et al. [14,15] examined the dimerization of halofluorescein dyes on Au(1 1 1) electrode in aqueous HClO4 solution by cyclic voltammetry and in situ scanning tunneling microscopic studies. Eosin yellowish [2 0 ,4 0 ,5 0 ,7 0 -tetrabromo-3 0 ,6 0 -di-hydroxyspiro[isobenzofuran-1(3H),9 0 -[9H]xanthen]-3-one] is a xanthene dye with strong absorption in the visible region. A number of well-characterized photophysical and photochemical properties of the dye molecules have enabled several researchers to study their spectral features in a variety of microenvironments. In this Letter, we have investigated the spectral characteristics of eosin in the presence of eight different sizes of gold nanoparticles. It has been found that smaller particles of gold stimulate J-aggregation of eosin on the surface of metal particles whereas any kind of aggregation amongst the dye molecules is not seen in the presence of larger particles. The changes in the spectral characteristics of eosin due to aggregation on metallic nanoparticles have been observed both in absorption and emission spectroscopy. Therefore, the present report addresses the behavior of eosin on the wide range of nanosized gold surfaces and probes the aggregation effect in a dye–metal nanocluster assembly.
COOH Br
Br O H
O
O Br
Br Eosin
2. Experimental 2.1. Reagents and instruments All the reagents used were of AR grade. Chloroauric acid (HAuCl4 Æ xH2O) and non-ionic surfactant, poly (oxyethylene) isooctyl phenyl ether (Triton X-100 or TX-100) were purchased from Aldrich. Molecular probe, eosin (Sigma) and ascorbic acid (SRL, India) were used as received. Doubled distilled water was used throughout the course of investigation. All chemical reactions were carried out in 1 cm wellstoppered quartz cuvettes. For the preparation of gold seeds, photoirradiation was carried out with a germicidal lamp (Philips, Holland, G15 T8 UVC, 15 W). The absorption spectra were measured in a Spectrascan UV 2600 spectrophotometer (Chemito, India). Fluorescence spectra were recorded with a Perkin–Elmer LS50B spectrofluorimeter equipped with a 9.9 W xenon flash lamp and a photo multiplier tube with S-20 spectral response. An excitation wavelength of 500 nm and slit width of 5/5 nm were used to record the spectra. Transmission electron microscopic (TEM) studies were carried out in a JEOL 4000 instrument at 400 kV. Samples were prepared by placing a drop of solution on a carbon coated copper grid. The temperature was 298 ± 1 K for all measurements. 2.2. Preparation of gold nanoparticles of variable sizes Gold nanoparticles were prepared by a two-step seedmediated (non-iterative) growth method. Seed particles were prepared by the UV-irradiation of a solution containing Au(III) ions (added as HAuCl4 Æ xH2O) in aqueous solution of TX-100. In practice, an aqueous solution (3 mL) of HAuCl4 (0.1 mM) was mixed with TX-100 (10 mM) in 1 cm quartz cuvette and photoirradiated for 30 min. The cuvette was kept at a distance of 3 cm from the light source. The particles formed by this method serve as seed particles (S). After 3–4 h appropriate amounts of seed particles and Au(III) ions were mixed in different molar ratios and to this solution ascorbic acid solution (0.13 mM) was added all at a time. This resulted in various sized gold particles depending on the seed particle to Au(III) concentration ratio. The particles formed by this method were allowed to stand for 24 h before their use for further studies. An in-depth study of the evolution and characterization of the gold particles has been discussed elsewhere [16]. The UV– vis spectral characteristics and sizes of the gold nanoparticles are summarized in Table 1. 2.3. Making gold–eosin assembly A known concentration of aqueous solution of eosin (7.5 lM) was mixed with the gold colloids (10 lM) of
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
7
Table 1 Synthetic conditions and UV–visible spectrophotometric and transmission electron microscopic characteristics of the prepared gold particles Sets
[seed Au] (M)
[Au(III)] (M)
kmax (nm)
Bandwidth (nm)
Particle diameter (nm)
S A B C D E F G
1.0 · 104 7.5 · 105 6.0 · 105 5.0 · 105 3.33 · 105 1.00 · 105 3.33 · 106 1.00 · 106
– 2.50 · 105 4.00 · 105 5.00 · 105 6.67 · 105 9.00 · 105 9.67 · 105 9.90 · 105
522 524 526 527 527 529 531 534
70 68 64 63 55 50 47 47
10
Exp.
different sets and the final volume of all the solutions were made to 3 mL. The mixtures were then allowed to stand for 12 h (for the reproducible dimerization) and subjected to absorption and emission measurements. All the mixtures contained 1 mM (300 lL of 10 mM) of TX-100 solution.
3. Results and discussion Aqueous solution of eosin (7.5 lM) exhibits a sharp monomeric absorption band in the visible region with a maximum at 516 nm (trace a) as shown in Fig. 1. In a micellar solution of TX-100 (1.0 mM), the absorption maximum is slightly red-shifted (kmax 518 nm) compared to that of the simple aqueous solution (trace b). However, significant alteration in the absorption characteristics of the dye (trace c) is seen in the presence of gold colloids (10 lM, set S). Keeping these observations in mind, we intend to study the size effect of gold nanoparticles on eosin taking the spectral features of the dye molecules into
a
0.6
b Absorbance (a. u.)
c 0.4
0.2
0.0 450
500 550 Wavelength (nm)
600
Fig. 1. Absorption spectra of eosin (7.5 lM) in (a) aqueous, (b) micellar solution (1 mM) and (c) in the presence of gold colloids (10 lM, set S). The gold colloid contains 1 mM of TX-100 solution.
12 14 22 31 46
Cal. 11 11.9 12.6 14.4 21.5 31.1 46.4
consideration. For this purpose, gold nanoparticles of eight different sizes have been employed. The size of the particles varies within the size range of 10–46 nm (sets S–G) as listed in Table 1. Interestingly, it is observed that the absorption characteristics of eosin show two distinct features for the two different size regimes. The amount of total gold (10 lM) is kept same for all the sets. The concentration of the dye is kept constant (7.5 lM) in all cases and all the solutions contain 1 mM of TX-100 solution. Fig. 2a shows the behavior of eosin in the absence and presence of different sets of gold particles. For gold particles (sets S, A, B, C) lying in the size range of 10–12 nm, a red shift (20 nm) in the absorption spectra of the dye are seen with a maximum at 538 nm. However, the band structures remain same in the presence of all sets of particles. As the size of the gold particles (sets D, E, F, G) become larger than 12 nm, the absorption maxima of eosin appears at 518 nm and the absorbance value slightly decreases. The optical properties of the dye molecules changes upon adsorption onto metal particles because the local field at the molecule increases by the dipole field of the resonant plasma sphere. The strong electron coupling between the dye and metallic particles results in the decrease in the intensity of the eosin absorption peak upon adsorption onto gold particles [8]. Eosin, in its monomeric form, is a strongly fluorescing dye (uf = 0.57). In the presence of gold particles, the emission behavior of the dye changes. Fig. 2b shows the fluorescence spectra of eosin in the absence and presence of gold nanoparticles. Eosin exhibits a broad emission band with a maximum at 544 nm (trace a*) in the presence of TX-100. Addition of colloidal gold (sets S, A, B, C) to the eosin solution lead to shifting of emission maxima to 557 nm as shown by the traces s, a, b, c respectively. In the presence of larger gold particles (sets D, E, F, G), the emission maxima again appear at 544 nm and the emission intensity decreases. The emission spectra of eosin in the presence of sets D, E, F, G are shown by traces d, e, f, g, respectively. A small variation in the intensity of eosin emission in the presence of different sets of gold particles is clearly resolved in
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S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
Fig. 2. (a) Absorption spectra of eosin (7.5 lM) in the absence and presence of gold particles of variable sizes (10 lM). (b) Emission spectra of eosin (7.5 lM) in (a*) absence and (S–G) presence of gold particles of variable sizes (10 lM).
the fluorescence spectroscopy (not distinguishable from the absorption spectroscopy). The appearance of red-shifted band at 538 nm in the extinction spectra can be attributed to the dimeric structure of the dye [6,9]. The red shift in the absorption maximum observed in Fig. 2a indicates that the eosin aggregates formed in colloidal suspensions are J-type. In fact, the J-aggregates are characterized by a redshifted absorption band relative to the monomer band as a result of exciton delocalization over the molecular building blocks of the aggregate [17]. The molecular exciton theory was proposed by McRae and Kasha [5] to afford the most satisfactory treatment of the electronic transitions in the dimeric form of the dye. This theory predicts that the excitonic singlet state of the dimer splits into two levels as a consequence of the two possible arrangements (in-phase and out-of-phase oscillation) of the transition dipoles of the chromophores in
the dimer. Since the exciton splitting depends on the oscillator strength of the transition, only the singlet excited state splits and the triplet excited state remains nearly degenerate. As a result of splitting, one level is now lower and the other is higher in energy than the corresponding monomer singlet excited state. Transitions from ground state to either excited state are possible and the number of bands actually observed depends on the geometry of the dimer. For parallel dimers (Htype aggregates), the transition to the lower energy excited state is forbidden and a single blue-shifted band is observed with respect to monomer. On the other hand, in case of head-to-tail dimers (J-type aggregates), the transition to the higher energy excited state is forbidden and the spectrum shows a single red-shifted band with respect to monomer. A schematic presentation of the H- and J-type aggregates and representative energy levels that control the excited state dynamics [3] are shown in Scheme 1. Kamat et al. [7] observed a blue shift in the absorption maximum upon addition of Rhodamine 6G to the gold colloids and concluded that the dimeric aggregates are of H-type. They pointed out a strong electronic coupling between the Rhodamine 6G molecules in the presence of gold colloids even at the very low (<105 M) dye concentrations. Since eosin is a strongly fluorescing dye, the aggregation effect is also seen in the emission spectra of the dye molecules. Absorption and emission spectroscopy are the techniques most widely used and yield the most important information about the aggregation of dye molecules in solution. Although Lo´pez Arbeloa reported the existence of trimer and higher aggregates in concentrated aqueous solution for such halofluorescein dyes [18], the presence of a single absorption band (kmax 538 nm) is indicative of the fact that only one kind of S1+ ic
S1 isc
_ S1
isc T1
T1
hv
E
E
hv //
hv /
E E
E
H-type
E
J-type S0
S0 Monomer
Dimer
Scheme 1. Representation of energy levels (S, singlet; T, triplet) of eosin (E) monomer and dimer that control the excited state dynamics [3].
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
aggregate exists on the surface of the gold colloids. It is necessary to recall that ascorbic acid is used for the growth of the gold particles and the absorption changes of eosin may occur due to some kind of interaction of the dye molecules with the unreacted ascorbic acid or the oxidation product of ascorbic acid (produced in the reaction medium during the reduction of gold chloride) molecules. However, no such changes are observed in the absorption spectrum when the same amount of ascorbic acid is added to eosin solution which indicates that shifting of absorption maximum is not due to unreacted ascorbic acid molecules. Furthermore, the shifting of dye maximum is also seen in the presence of seed particles (S) that contain no ascorbic acid or the oxidation product of ascorbic acid molecules. Fluorescence decay measurements (kex 405 nm) of excited dye molecules exhibit lifetimes of 3.4 and 3.2 ns in micellar environments and the presence of gold particles indicating the attachment of the probe molecules on the gold surface. As the dye aggregation does not occur at such a low concentration of the dye in the absence gold particles, it is reasonable to assume that intermolecular interactions between the eosin molecules on the gold surface is responsible for the dimerization process. Since in the presence of gold colloids the dye kmax appears at 538 nm, it is suspected that the gold nanoparticles can contribute to the absorption spectrum of the dye. But it was noted that for such a low concentration of the gold colloid (10 lM), the optical density is negligibly small and can be neglected compared to dye absorption. Moreover, the shifting of dye emission maximum is also seen in the fluorescence spectrum of the dye, but the gold nanoparticles are non-fluorescent. Therefore, it can be concluded that the shifting of dye absorption is not due to the interaction with the ascorbic acid molecules or to significant optical contribution due to the gold particles. The size regime dependence of the gold nanoparticles on the dimerization of eosin molecules can be rationalized by correlating the structure of the nanoparticles in terms of their surface parameters. The noticeable shift of the absorption and emission maximum of eosin is seen only in the presence of smaller gold nanoparticles that for a specified size regime but not for particles of larger dimensions. As the gold concentration is same for all sets of particles, the larger is the particle the lower is the concentration of the particles and the surface area is correspondingly lower. The number of dye molecules associated with each gold nanoparticle was determined by calculating the particle concentration in the gold colloids. At first, the aggregation number was calculated using the formula: NAu = (59 nm3)(p/6)(DMS)3, where DMS is the mean diameter of the particles. Thus, a gold particle of 10 nm diameter is composed of 30 867 gold atoms and each particle corresponds to 23 163 eosin mole-
9
cules. The surface-to-volume atomic ratio (a) of metal particles is defined by, a = 3d/R, where d and R denote the atomic diameter and particle size respectively. ˚ for atomic gold, the surfaceAssuming d = 3.58 A to-volume atomic ratio, a = 0.1074 which indicates amongst the 30 867 gold atoms in a 10 nm gold particle, 3315 atoms are located at the surface. It has been seen that the formation of multilayer of the dye molecules on metallic nanoparticles depends on dye concentration, pH and hydrogen bonding [19]. Thus, a 10 nm gold particle with a surface area of 314 nm2 and 3315 atoms on the surface accommodate large number of eosin molecules through the formation of multilayers of the dye molecules which contain functional moieties capable of forming hydrogen bonds. The number of eosin molecules (the dye concentration remaining same for all sets) corresponding to the gold surface increases with increase in particle size as the number of particles decreases in solution. Thus, higher dye concentrations per gold nanoparticle existing in solutions through D–G and is expected to induce more extensive dimerization than in colloids S–C. The observed dye aggregation in the presence of smaller particles can be interpreted by considering that dye molecules induce intercluster interactions within the smaller particles and such aggregates which bring adsorbed dye molecules closer facilitate dimer formation in the assembly of the dye molecules [20]. Since, the molar surface area increases with decreasing particle size, there results an increase in molar free energy of the small particles compared to larger ones and thus, the nanoparticle aggregation predominates in colloids with smaller particles after addition of the dye [21]. So, aggregation of particles is the cause and dimerization of dye molecules is the effect. Transmission electron micrographs (Fig. 3) of the gold colloids (set S) before and after addition of the dye molecules show that the gold particles are aggregated in solution after addition of the dye molecules. During the formation of gold aggregates, the close packing of the dye molecules around the gold surface induce interparticle interactions and dipolar interaction between the two bromine atoms from two eosin molecules is responsible for the dimer formation [14,15]. Fig. 4 shows an increase in the intensity of 538 nm peak, i.e., increases the concentration of the dimer with increase in the concentration of gold colloids (set S). Inset shows a linear increase of the dimer concentration with increase in the concentration of the gold colloids indicating that extent of dimerization increases with increased number of gold particles in the aggregate. As the dimerization of the dye molecules occurs due to aggregation amongst the gold particles, an aging (12 h) of the gold–probe assembly is required for the reproducible dimerization of the dye molecules. The formation of dye aggregates only in the presence
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S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11
Fig. 3. Transmission electron micrograph of the gold particles (set S) (a) before and (b) after addition of the dye molecules.
cles due to their high molar surface free energy that the surface phase possesses. The formation of dye aggregates modifies the absorption and emission spectrum and the photophysical properties of the dye. Therefore, the aggregation phenomenon of eosin can be successfully exploited as a marker to label different size regimes of gold nanoparticles utilizing absorption and fluorescence spectroscopy.
0.6
0.4
Absorbance (a. u.)
Absorbance (a.u.)
0.52
0.48
0.44
0.40 0
2
4
6
8
10
[Au] (µM)
0.2
Acknowledgments
0.0 400
450
500 Wavelength (nm)
550
600
Fig. 4. Extinction spectrum of eosin (7.5 lM) in presence of different concentration of gold particles (set S). Inset shows the plot of absorbance (at 538 nm) as a function of gold concentration.
of smaller particles indicates that there is a dramatic increase in surface energy of the particles as the diameter decreases below 12 nm. Recently, it has been reported from our laboratory that fluorescence quenching of 1-methylaminopyrene and the catalytic reduction of eosin on the surface of gold nanoparticles are also size regime dependent phenomena [16,22].
4. Conclusion This Letter describes a straightforward experimental but spectroscopic verification of size effect of nanoscale materials on the dimerization of eosin molecules. The dye molecules undergo dimerization when bound to smaller gold particles even at very low concentrations but remain in its monomeric form in the presence of larger gold nanoparticles. The major factor driving the dimerization process is the aggregation of smaller parti-
We gratefully acknowledge financial support from DST and CSIR, New Delhi. We are thankful to Prof. Kankan Bhattacharyya for providing DST funded Femtosecond Facilities for lifetime measurements.
References [1] M. Kawasaki, T. Sato, Y. Yoshimoto, Langmuir 16 (2000) 5409. [2] R. Shinozaki, T. Nakato, Langmuir 20 (2004) 7583. [3] C.P. Nasr, D. Liu, S. Hotchandani, P.V. Kamat, J. Phys. Chem. 100 (1996) 11054. [4] P. Xu, H. Yanagi, Chem. Mater. 11 (1999) 2626. [5] E.G. McRae, M. Kasha, in: L. Augenstein, R. Mason, B. Rosenberg (Eds.), Physical Processes in Radiation Biology, Academic Press, New York, 1964, p. 23. [6] J. Hranisavljevic, N.M. Dimitrijevic, G.A. Wurtz, G.P. Wiederrecht, J. Am. Chem. Soc. 124 (2002) 4536. [7] N. Chandrasekharan, J. Hu, G. Jones II, P.V. Kamat, J. Phys. Chem. B 104 (2000) 11103. [8] M. Kerker, J. Colloid Interface Sci. 105 (1985) 297. [9] N. Kometani, M. Tsubonishi, T. Fujita, K. Asami, Y. Yonezawa, Langmuir 17 (2001) 578. [10] O. Stenzel, A. Stendal, A.C. Voigtsberger, Solar Energy Mater. Sol. Cells 37 (1995) 337. [11] K. Goerge Thomas, P.V. Kamat, J. Am. Chem. Soc. 122 (2000) 2655. [12] O.V. Makarova, A.E. Ostafin, H. Miyoshi, J.R. Norris, D. Miesel, J. Phys. Chem. B 103 (1999) 9080. [13] A.C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27. [14] G.-J. Su, S.-X. Yin, L.-J. Wan, J.-C. Zhao, C.-L. Bai, Chem. Phys. Lett. 370 (2003) 268.
S.K. Ghosh et al. / Chemical Physics Letters 412 (2005) 5–11 [15] G.-J. Su, S.-X. Yin, L.-J. Wan, J.-C. Zhao, C.-L. Bai, Surf. Sci. 551 (2004) 204. [16] T.K. Sau, A. Pal, T. Pal, J. Phys. Chem. B 105 (2001) 9266. [17] D.A. Higgins, P.J. Reid, P.F. Barbara, J. Phys. Chem. 100 (1996) 1174. [18] I. Lo´pez Arbeloa, J. Chem. Soc., Faraday Trans. 2 (77) (1981) 1725.
11
[19] E.W.L. Chan, D.C. Lee, M.K. Ng, G. Wu, K.Y.C. Lee, L. Yu, J. Am. Chem. Soc. 124 (2002) 12238. [20] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [21] H. Zhang, R.L. Penn, R.J. Hamers, J.F. Banfield, J. Phys. Chem. B 103 (1999) 4656. [22] S.K. Ghosh, A. Pal, S. Kundu, S. Nath, T. Pal, Chem. Phys. Lett. 395 (2004) 366.